As Advertising Week wound down on Friday, a dark cloud lingered on the horizon that could leave Madison Avenue struck by lightning.

It’s a controversial congressional tax proposal released last November that would cut in half the deduction large advertisers can take in a year.

“The draft tax reform proposals would drastically cut the deduction. Companies that spend $1 million or more on advertising in a year would be allowed to deduct only 50 percent of their advertising costs,” according to the Association of National Advertisers (ANA), which is lobbying to kill the change.

Advertising is a critical part of New York’s economy. It generates some half-trillion dollars of annual economic activity, about 20 percent of the state’s economic output, an ANA official says.

The ANA official said it could cost New York City thousands of jobs.

“For every dollar spent on advertising,” said Daniel Jaffe, group vice president of the ANA, “nearly $22 would be generated in the economy. That means that the state of New York alone could see the loss of billions of dollars in economic output and place at risk thousands of jobs if the proposed tax on advertising is enacted.”

The ANA sent its letter to Sen. Charles Schumer, a member of the Senate Finance Committee.

Schumer (D-NY) said the proposal is DOA in the short term. Still, he promised to watch “like a hawk for any threats that may arise.”

“It is unfair, illogical and counterproductive to treat business advertising costs any different from other ordinary and necessary business expenses,” Schumer told The Post, “and I will vigorously oppose any proposals in this vein.”

The measure is part of a comprehensive tax reform plan before Congress’ key tax-writing body, the House Ways and Means Committee, which could say the other 50 percent of the deduction should be written off over several years.

Congress’ Joint Committee on Taxation estimates that spreading out the deduction over years would generate $169 billion for the government through the year 2023.